We now have a much simpler DataSourceAutoConfiguration that binds to whatever
DataSource concrete type it finds at runtime. To be able to quickly switch between
Hikari and the other types of DataSource there's a minute shim for translating
the common properties (username, password, url, driverClassName), but actually
only url is different. The shim and also DataSource initialization is supported
through DataSourceProperties, but the other native properties get bound directly
through the concrete runtime type of the DataSource.

The /configprops endpoint works (and is exposed in the actuator sample).

Fixes gh-840, fixes gh-477, see also gh-808.